How to analyse the structure of radicals: electron spin
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص975-976
2025-07-28
515
How to analyse the structure of radicals: electron spin
resonance for the last few pages we have been discussing the species we call radicals without offering any evidence that they actually exist. Well, there is evidence, and it comes from a spectroscopic technique known as electron spin resonance, or ESR (also known as EPR, electron paramagnetic resonance). ESR not only confirms that radicals do exist, but it can also tell us quite a lot about their structure. Unpaired electrons, like the nuclei of certain atoms, have a magnetic moment associated with them. Proton NMR probes the environment of hydrogen atoms by examining the energy difference between the two possible orientations of their magnetic moments in a magnetic f i eld; ESR works in a similar way for unpaired electrons. The magnetic moment of an electron i s much bigger than that of a proton, so the difference in energy between the possible quantum states in an electron field is also much bigger. This means that the magnets used in ESR spectrometers can be weaker than those in NMR spectrometers, usually about 0.3 tesla; even at this low fi eld strength, the resonant frequency of an electron is about 9000 MHz (for comparison, the resonant frequency of a proton at 9.5 tesla is 400 MHz; in other words, a 400 MHz NMR machine has a magnetic fi eld strength of 9.5 tesla). But there are strong similarities between the techniques. ESR shows us, for example, that unpaired electrons couple with protons in the radical. The spectrum below is that of the methyl radical, Ch3 • .The 1:3:3:1 quartet pattern is just what you would expect for coupling to three equivalent protons; coupling in ESR is measured in millitesla (or gauss; 1 gauss = 0.1 mT), and for the methyl radical the coupling constant (called aH) is 2.3 mT.
ESR hyperfine splittings (as the coupling patterns are known) can give quite a lot of inform ation about a radical. For example, here is the hyperfine splitting pattern of the cyclo heptatrienyl radical. The electron evidently sees all seven protons around the ring as equivalent, and must therefore be fully delocalized. A localized radical would see several different types of proton, resulting in a much more complex splitting pattern.
Even the relatively simple spectrum of the methyl radical tells us quite a lot about its structure. For example, the size of the coupling constant aH indicates that the methyl radical is planar; the tri-fluoromethyl radical is, on the other hand, pyramidal. The oxygenated radicals •CH2OH and •CMe2OH lie somewhere in between. The calculations that show this lie outside the scope of this book.
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